CHANGES IN MICROBIAL BIOMASS AND ORGANIC-MATTER LEVELS DURING THE 1STYEAR OF MODIFIED TILLAGE AND STUBBLE MANAGEMENT-PRACTICES ON A RED EARTH

Farming practices involving stubble burning and excessive tillage in A ustralia have led to losses of organic matter from the soil. Crop resi due retention and reduced tillage practices can reverse these trends, but changes in organic matter levels are evident only after a long ter m. Microbial biomass (MB), the living portion of soil organic matter, responds rapidly to changes in soil and crop management practices. We evaluated changes in microbial biomass and microbial activity in the f irst year following the modification of stubble management and tillage practices on a red earth near Harden, New South Wales. Following an o at crop harvested late in 1989, seven treatments involving stubble and tillage management were applied in February 1990. Wheat was planted i n May 1990. Measurements of total organic carbon (C) and total nitroge n (N) in the top 15 cm of soil indicated no significant changes after 1 year, although there was a significant effect on the distribution of C and N. However, significant changes in MB were observed in the firs t year. Microbial biomass C in the top 5 cm of the soil ranged from 25 to 52 g C m-2 and these levels dropped by 50% or more with each 5 cm depth. Implementation of treatments altered MB, particularly in the to p 5 cm where MB-C and MB-N were significantly greater in stubble-retai ned than in the top 5 cm where MB-C and MB-N were significantly greate r in stubble-retained than in the stubble-burnt treatments, and in the direct drill treatment than in the stubble-incorporated treatment. Mi crobial biomass in soil increased during the growth of wheat in all tr eatments, but this was slower in the standing stubble-direct drill tre atment, probably due to the delay in the decomposition of stubble. Mic robial respiration, which was concentrated in the surface 5 cm of soil in all treatments, was greatest in the direct drill treatments. Micro bial activity below 5 cm was higher with stubble incorporation than wi th direct drill. Specific microbial activity (microbial respiration pe r unit MB) had the greatest response to tillage at 10-15 cm depth. Mic robial quotients (MB as a percentage of C or N) responded to changes i n tillage but not significantly to stubble retention. Our studies, dur ing the first year following the modification of stubble management an d tillage practices, suggested that changes in MB and microbial activi ty may be sensitive and reliable indicators of long-term changes in or ganic matter in soils.